You get what you pay for. Your Dentist will take impressions of your teeth and have a night guard made from your mouth's molds. So only you would be able to wear your night guard comfortably.
The type you buy "over the counter" are one size fit most that you try to fashion to your mouth???
Yes the night guard will help. It keeps your teeth apart. Clenching and grinding exerts tons of pressure. When you do grind, the biting forces are distributed more evenly across the biting surface of the guard. Jaw muscles are not over worked. Prevents breaking filling or teeth. Less pressure on the TMJ joint. Some people experience fewer headaches.